`[NiCl_(4)]^(2-)` is paramagnetic while `[Ni(CO)_(4)]` is diamagnetic though both are tetrahedral Why?

Explanation

Ni in `[NiCl_4]^(2-)` is in +2 oxidation state and its configuration is :

`Cl^(-)` being a weak ligand, cannot pair up the electrons in 3d orbitals. Hence, it is paramagnetic. In `[Ni(CO)_4], Ni` is in zero oxidation state and its configuratiion is :

In the presence of strong CO ligand, the 4s electrons shift to 3d to pair up with 3d electrons. Thus, `[Ni(CO)_4]`, is diamagnetic due to the absence of unpared electrons.